Personally, my order is Haaland > Lukaku > Kane
Getting any of them is a big win, but the positives I see with Lukaku are:
1) Cheapest out of all 3, even though he said he wants to stay at Inter, I'm sure if City offered them 80m, Inter could be tempted to it due to their financial problems. Kane and Haaland seem unreasonable in that regard.
2) He is a big unit that's for sure, but he is also surprisingly fast and a good dribbler, advantages over Kane (and probably Haaland).
3) His passing is pretty good, not Kane good, but I think good enough for a "Pep Team" (that loses more meaning every year).
4) His ability to drift to the wings is more suited than Kane or Haaland (for now), Kane is better at dropping deep and playing as a 10, but I don't think we need that as much, we have so many players who can play as a 10
5) The obvious one, he and KdB are around the same age and know each other's game well enough at the international level.
Edit: I think there is a good idea in playing Torres or Jesus next to Lukaku and have them play a similar role like Lautaro at Inter. Could suit both those players if they don't have to play as a "real" lone striker, but in a more flexible system where they can drift wide and drop deep.
------------------------Ferran/Jesus----------------Lukaku------------------
Foden-----------------Gundogan----------------KdB---------------------Cancelo/Sterling
------------------------------------------Rodri
------------Laporte/Ake------------Dias---------------Stones/Walker